This invention relates to an improvement in techniques for the manufacture of hollow, fiber-reinforced articles, and more particularly it pertains to a process for forming a resin-rich inner coating in a pressure vessel to eliminate possible wicking of chemicals through the wall of the vessel. This invention is an improvement in the type of molding technique described in U.S. Pat. No. 2,977,269 to Nerwick and U.S. Pat. No. Reissue 25,241 to Randolph. According to the techniques described in those patents, hollow, cylindrical, fiber-reinforced plastic tanks have been successfully and economically pressure-molded. Generally, those techniques include the steps of laying up a fiber form within a rigid mold casing in the approximate form of the desired article. A complete fiber form may include preformed, matted fiber end wall caps which telescope into a laid up cylindrical sidewall portion. More desirably, the fiber form comprises a preformed tube having a closed end, as described in U.S. Pat. No. 3,911,965 and a single preformed end cap closing the open end of the preform.
An expandable bag or envelope, which will define the interior shape of the finished article, is positioned within the fiber form in the mold. Rigid casing caps, which shape the end walls of the final article, are clamped to the ends of the mold casing to enclose the laid-up form. With the fiber form and mold thus assembled, the fiber form is placed under a suitable moderate pressure by expanding the bag to hold it in place against the mold. The fiber form is then permeated with a thermosetting resin or the like. The bag, besides defining the interior surface of the molded tank and determining the proper fiber-to-resin ratio, compresses the fiber form in such a manner as to avoid migration of fibers with resin flow and resulting destruction of the lay-up. The bag is further expanded to compress the lay-up and to evenly distribute the resin throughout the lay-up. The mold is then heated to cure the resin, the bag is removed from the interior of the molded tank, and the finished molded tank is removed from the mold.
Tanks produced in accordance with the aforementioned prior art techniques have been widely employed as water softener tanks, swimming pool filter tanks, and the like. One problem exists, however, which is not serious when the tanks are employed to hold water, but which is serious when the tanks are employed to hold corrosive chemicals. That problem involves a phenomenon known as "wicking," wherein fibers may communicate with the inner surface of the tank and the outer surface of the tank. In such an instance, the fluid contained in the tank may be drawn along the fiber by capillary action so that the fluid migrates through the sidewall of the tank to thereby degrade the plastic to a degree causing failure of the tank and forms a finely beaded liquid coating on the exterior surface.
In view of this problem, attempts have been made to line the interior of the tank with a resin-rich or gel coat. This involves a separate step in the process, since the operation is performed after the tank is cured and removed from the mold. Furthermore, such an operation results in a secondary bond between the molded sidewall of the tank and the coating, which creates the possibility of delamination of the coating during use.